In order to drive an alternating current (AC) motor by means of a direct current power source, a driving method in which an inverter is employed is being adopted. Switching of an inverter is controlled by an inverter driver circuit, so that, based on the result of comparison between a voltage instruction and a triangular wave carrier, a pulse width modulation (PWM) voltage which is output from the inverter and applied to the AC motor is controlled, for example.
Overmodulation PWM control has been proposed so as to increase the amplitude of a fundamental wave component of the pulse width modulation voltage to be applied to the AC motor (see JP 7-194130A, JP 7-79570 A, and so on). In the overmodulation PWM control, the pulse width modulation voltage to be applied to the AC motor is controlled based on the result of comparison between a voltage instruction in which an amplitude exceeds a peak value of a triangular wave carrier, and the triangular wave carrier.
In overmodulation PWM control in which a phase voltage instruction amplitude exceeds a peak value of a triangular wave carrier, the amplitude of a fundamental wave component of the pulse width modulation voltage output from the inverter does not increase linearly with respect to a linear increase in the phase voltage instruction amplitude and is smaller than the phase voltage instruction amplitude. To address this disadvantage, in JP 7-194130 A, a voltage instruction E is calculated from the following formula (1) to perform linear compensation of the voltage instruction E. In formula (1), A represents a percent modulation and Ed represents a direct current (DC) voltage to be input to the inverter.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      E          =                                    1              2                        ⁢                          {                                                                    sin                                                              -                      1                                                                                    A                                    ⁢                                      1                    A                                                  +                                                      1                    -                                          1                                              A                        2                                                                                                        }                        ⁢                          E                              ma                ⁢                                                                  ⁢                x                                                    ⁢                                  ⁢                  Here          ,                                          ⁢                                    E                              ma                ⁢                                                                  ⁢                x                                      =                                                            2                                π                            ⁢                              E                d                                                                        (        1        )            
However, according to formula (1), a waveform (continuous values) which is obtained by clamping a phase voltage instruction value having a sinusoidal wave with half the direct current voltage to be input to the inverter is obtained. Accordingly, when the number of triangular wave carriers per one cycle of a voltage instruction is sufficiently large, a pulse which is output by PWM is substantially equal to the continuous values of waveform described above and therefore formula (1) is substantially satisfied, whereas when the number of triangular wave carriers per one cycle of a phase voltage instruction is reduced, formula (1) cannot be satisfied. Consequently, the fundamental wave amplitude corresponding to the phase voltage instruction amplitude cannot be obtained, resulting in a reduction of controllability of overmodulation PWM control.